Relay devices are known electrically controllable, high current switches that are used in a variety of applications. In one such application, relay devices are typically used to couple direct current power supplies to base station transmitters at base sites of communication systems. In another application, relay devices may be used to couple a high current supply to a loading circuit, for example in a power transmission system or an automobile starter circuit.
Relay devices are known to comprise a coil and a set of contacts. In a typical configuration, the relay device is open--thereby prohibiting current flow--when the relay contacts are open, and closed--thereby permitting current flow--when the relay contacts are closed. Consequently, an electric control circuit is used to open and close the relay contacts (i.e., control the relay device) depending on whether the relay device is to be opened or closed, respectively. In a typical application, a relay device is controlled electrically to allow a loading circuit to be enabled and disabled--for example, when engaging and disengaging, respectively, the starter of an automobile engine.
In general, relay device operation occurs as follows. When an initially open relay device is to be closed, the control circuit enables a transistor circuit coupled to the relay coil. The transistor circuit enables an amount of current specified by the relay manufacturer to flow through the relay coil from a power supply coupled to the relay coil. The current in the relay coil induces a magnetic field around the coil. The magnetic field provides the force necessary to close the relay contacts, thereby allowing the relay contacts to provide a current path between the power supply and a loading circuit. When the relay device is to be re-opened, the control circuit disables the transistor circuit to remove the current in the relay coil, thereby removing the magnetic field and opening the relay contacts. Therefore, in the prior art, whenever the relay is closed, the current necessary to close the relay contacts continually flows through the relay coil and is determined by the resistance of the relay coil. In a typical situation, the current necessary to close the relay contacts is significant (e.g., 500 milliamps) and results in substantial power dissipation (e.g., 13 Watts at 26 Volts) in the relay coil, especially since the relay coil is not typically heat sunk and is physically small (e.g., 7.5 centimeters long by 5 centimeters in diameter). Excessive dissipation in the relay coil reduces the mean-time-to-failure of the relay coil, thereby decreasing the reliability of the relay device and, often, the loading circuit coupled to the relay device.
Therefore, a need exists for a method and apparatus for drawing current through a relay device that closes and maintains closure of the relay device while minimizing power dissipation in the relay coil. Further, such a method and apparatus that provide redundant control of the relay device would be an improvement over the prior art.